Prehistoric World

Despite the general acceptance of a diversion in the lines between the ancestors of humans and their chimpanzee cousins around 7 million years ago, the final split may have taken place far more recently than was previously thought.

A detailed analysis of human and chimp DNA suggests that the lines finally diverged less than 5.4 million years ago. The finding, which has been published in the journal Nature, is about 1-2 million years later than the fossils themselves have indicated.

A US team says that its results hint at the possibility that interbreeding occurred between the two lines for thousands, and even millions, of years. This hybridisation would have been important in swapping genes for traits which allowed the emerging species to survive in their environments, explained the scientists, who are affiliated to the Broad Institute of MIT and Harvard and the Harvard Medical School.

And it underlines, they believe, just how complex human evolution has been.

'This is a hypothesis; we haven't proved it but it would explain multiple features of our data,' said David Reich, assistant professor of genetics at the Harvard Medical School and an author on the Nature paper. 'The hypothesis is that there was gene flow between the ancestors of humans and chimpanzees after their original divergence.'

'So, there may have been an original divergence and a separation for long enough that the species became differentiated - for example, we may have adapted features such as upright walking - and then there was a re-mixture event quite a while after; a hybridisation event,' he told the Science in Action programme on the BBC World Service.

Gene swapping

Humans and chimps contain DNA sequences which are very similar to each other; the differences are due to mutations, or errors, in the genetic code which have occurred since these animals diverged onto separate evolutionary paths.

Shown here is the cranium of Sahelanthropus tchadensis, one of perhaps several species on the human side of the human-chimpanzee divide which may still have interbred with an early chimpanzee species at a time at which the two lines were still very similar - perhaps no more different than modern humans and Neanderthals who certainly did interbreed (a, facial view. b, lateral view. c, dorsal view. d, basal view)

By analysing where these differences occur in the animals' genomes, it is possible to gain a level of insight into the two species' histories - the timing of key events in their evolution. Scientists have been able to do this for some time but the recent projects to fully decode the two primates' genomes have provided details which have taken this type of study to a more advanced level

The US investigation indicates that human and chimp lines finally divided no more than 6.3 million years ago and probably less than 5.4 million years ago. It is a problematic finding because of our current understanding of early fossils, such as the famous Toumaï specimen which was uncovered in Chad in 2001.

Toumaï (Sahelanthropus tchadensis) was thought to be right at the foot of the human family tree. It is dated between 6.5 and 7.4 million years ago. In other words, it is older than the point of human-chimp divergence seen in the genetic data.

'It is possible that the Toumaï fossil is more recent than previously thought,' said Nick Patterson, a senior research scientist and statistician at the Broad Institute of MIT and Harvard, and lead author on the Nature paper. 'But if the dating is correct, the Toumaï fossil would precede the human-chimp split. The fact that it has human-like features suggests that human-chimp speciation may have occurred over a long period with episodes of hybridisation between the emerging species.'

Commenting somewhat broadly on the research, Daniel Lieberman, a professor of biological anthropology at Harvard, told the Associated Press: 'It's an extremely clever analysis. My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates, not to put it too crudely.'

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